Lock minutes in small cupboard as reading aid (6,4)
I believe the answer is:
stress mark
'reading aid' is the definition.
(mark showing how a word should be read out)
'lock minutes in small cupboard' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'minutes' becomes 'm'.
'in' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cupboard' becomes 'ark' (as in the Ark of the Covenant).
'tress'+'m'='tressm'
's'+'ark'='sark'
'tressm' put into 'sark' is 'STRESS-MARK'.
'as' is the link.
(Another definition for stress mark that I've seen is " It helps one to pronounce".)